This invention relates generally to superconducting analog-to-digital converters and, more particularly, to high-performance superconducting analog-to-digital converters employing bidirectional counters.
High-performance analog-to-digital (A/D) converters are required in a variety of commercial and military electronic devices. Two important measures of an A/D converter's performance are speed, which is the number of samples generated by the A/D converter per second, and resolution, which is the smallest increment of change that can be detected in an analog signal. Superconducting technology is particularly well suited for performing high-speed, high-resolution A/D conversion because superconducting Josephson tunnel junctions, which provide the basic switching functions for many superconducting electronic devices, including superconducting A/D converters, possess a unique combination of speed, sensitivity, and periodic response characteristics. Josephson junctions are typically combined with one or more inductive loads in a superconducting loop to form a logic circuit called a superconducting quantum interference device (SQUID).
U.S. Pat. No. 5,012,243 to the present inventor discloses a high-speed, high-resolution superconducting analog-to-digital converter having a double-junction SQUID quantizer and a bidirectional binary counter employing n stages of four-junction SQUID flip-flops, where n is the number of bits of the digital output. The SQUID quantizer digitizes an analog input current by generating a measurable voltage pulse across one of the Josephson junctions each time the current changes by an incremental amount. These up-count and down-count voltage pulses are of the same polarity and are generated on two different output lines. The bidirectional binary counter algebraically counts the voltage pulses, increasing the binary count when upcount pulses are received and decreasing the binary count when down-count pulses are received. The incremental change in current required for a voltage pulse is equal to a single flux quantum (2.07.times.10.sup.-15 weber) divided by the inductance (measured in Henries) of the quantizer. Therefore, the incremental change in current is very small, thus providing both high-speed and high-resolution A/D conversion.
A third measure of an analog-to-digital converter's performance is power consumption, which is particularly important in superconducting electronic devices such as infrared focal plane arrays. Ideally, the power consumption of an entire channel of an infrared focal plane array should be limited to about 10 microwatts. This limits the power consumption of the A/D converter in each channel, which may have up to 12 bits or more of digital output, to about one microwatt. However, the analog-to-digital converter described above, as well as other high-performance superconducting A/D converters, cannot meet this stringent power requirement because of the power drain caused by the gate bias of the counter. Accordingly, there has been a need for a high-performance superconducting analog-to-digital converter having low power consumption. The present invention fulfills this need.